1. Field of the Invention
The present invention relates to a display device capable of inputting information from a screen by light.
2. Description of the Related Art
In recent years, a liquid crystal display device has been widely used for various instruments such as a cellular phone and a notebook computer. The liquid crystal display device includes a display unit having pixels on respective intersections of plural scan lines and plural signal lines, which are wired so as to intersect each other, a drive circuit which drives the scan lines, and a drive circuit which drives the signal lines. Each of the pixels includes a thin-film transistor, a liquid crystal capacitor, and an auxiliary capacitor. In general, the display unit is formed on an array substrate made of glass. Owing to development of circuit integration technologies in recent years, it has become possible to also form a part of the drive circuits on the array substrate, and weight saving and miniaturization of the entire liquid crystal display device have been achieved.
Moreover, as disclosed in Japanese Patent Laid-Open Publication No. 2004-318819, there is developed a display device which arranges optical sensors in pixels, thus making it possible to input information from a screen by light.
In this type of display device, for example, photodiodes are used as the optical sensors, and in the respective pixels, capacitors are connected to the photodiodes. Then, an amount of charge of each capacitor is changed in response to a change of an amount of received light in each photodiode, a voltage between both ends of the capacitor is detected, and data of an imaged image is thus generated.
As applications of the display device as described above, ones having a touch panel function and a digitizer function have been proposed. The touch panel function is to input information by detecting a shadow made by an object such as a finger projected on the screen. The digitizer function is to input information by detecting light irradiated from a light-emitting object such as a light pen.
However, the conventional display device including the light input function has a problem that it is difficult to determine whether or not the object such as the finger and the light pen has contacted the screen, and to calculate a coordinate position of the object accurately.
In particular, in a technique of utilizing the shadow made by the object, when an ambient environment is dark, the shadow made by the object approaching the screen cannot be identified accurately. Accordingly, accuracy in the information input drops. Moreover, also when the ambient environment is too light, the accuracy in the information input drops.
Moreover, the imaged image obtained by inputting the light is an image of a binary monochrome gradation, and accordingly, there are limitations on determination accuracy in the case of determining reflected light from the object on the screen and other light in a binary manner.
It is an object of the present invention to enhance the accuracy in determining whether or not the object has contacted the screen.
It is another object of the present invention to enhance the accuracy in calculating the coordinate position of the object on the screen.
It is still another object of the present invention to determine the reflected light from the object on the screen and the other light in the binary manner accurately.